Method for producing a thin-walled molded article of a polyacetal resin

ABSTRACT

PCT No. PCT/JP93/01651 Sec. 371 Date Jun. 13, 1994 Sec. 102(e) Date Jun. 13, 1994 PCT Filed Nov. 12, 1993 PCT Pub. No. WO94/09676 PCT Pub. Date May 11, 1994.The present invention provides a method for producing a thin-walled molded article of a polyacetal resin comprising adding 10 to 1000 ppm of boron nitride having an average particle diameter of 1 to 10  mu m and a content of diboron trioxide of 0.005 to 1.0% by weight to a polyacetal resin and molding an article having a maximum wall thickness of 0.2 to 1 mm by injection molding. The present invention can provide a molded article of a polyacetal resin having little warpage at a high temperature and humidity.

TECHNICAL FIELD

The present invention relates a method for producing a thin-walledmolded article of a polyacetal resin having excellent dimensionalstability to temperature and humidity changes.

BACKGROUND ART

A polyacetal resin is widely used as an engineering resin because of itsexcellent mechanical strength, fatigue resistance, electrical propertiesand the like.

However, since the polyacetal resin has high crystallizability, it hasdrawbacks that a molded article of it undergoes warpage or sink marks byheat dissipation immediately after injection molding and after heatdissipation (cooling), the molded article further undergoes dimensionalchanges such as warpage when it is exposed to a high temperature andhumidity. Particularly, in case of a thin-walled molded article,remarkable warpage appears on it.

The present invention has been completed based on finding that warpageof a thin-walled molded article is reduced when the amorphous layer,which is the skin layer of the molded article, is made as thin aspossible, compared to the total thickness, by using a specific boronnitride.

Japanese Patent Publication Unexamined No. 94185/1971 discloses thatboron nitride is added to polyoxymethylene. However, it does notdisclose the specific boron nitride used in the present invention andthe thin-walled injection molded article of the present invention.

An object of the present invention is to provide a thin-walled moldedarticle of a polyacetal resin, whose dimensions change very little at ahigh temperature and humidity.

DISCLOSURE OF THE INVENTION

The present invention relates to a method for producing a thin-walledmolded article of a polyacetal resin comprising adding 10 to 1000 ppm ofboron nitride, having an average particle diameter of 1 to 10 μm and acontent of diboron trioxide of 0.005 to 1.0% by weight, to a polyacetalresin and molding an article having a maximum wall thickness of 0.2 to 1mm by injection molding.

In the present invention, the thin-walled molded article is a moldedarticle having a maximum wall thickness of 0.2 to 1 mm. The moldingmethod of the present invention is particularly useful for producing athin-walled molded article having a maximum wall thickness of 0.25 to0.6 mm.

The polyacetal resins to be used in the present invention include apolyacetal resin homopolymer consisting substantially of oxymethyleneunits, produced by using a formaldehyde monomer or a cyclic oligomersuch as a trimer (trioxane), tetramer (tetraoxane) and the like as a rawmaterial, and a polyacetal resin copolymer comprising 0.1 to 2% byweight of a oxyalkylene unit having 2 to 8 carbon atoms, produced byusing the above raw material and cyclic ethers such as ethylene oxide,propylene oxide, epichlorohydrine, 1,3-dioxolane, a formal of a glycol,a formal of a diglycol and the like. The polyacetal resin furtherincludes a branched polyacetal copolymer having a branched molecularchain and a block copolymer comprising 50% by weight or more of anoxymethylene repeating unit and other component blocks. In the presentinvention, if necessary, the above polyacetal resins can be used incombination. However, the polyacetal resin homopolymer or the polyacetalresin copolymer is preferably used alone in view of dimensionalstability. The polyacetal resin homopolymer is more preferable.

The MFR measured under ASTM D1238-57 E condition of a polyacetal resinis preferably 40 g/10 min. or more, more preferably 60 g/10 min. ormore, most preferably 70 to 100 g/10 min.

The present invention is characterized in injection molding a polyacetalcomprising a specific boron nitride.

An average particle diameter of boron nitride to be used in the presentinvention is 1 to 10 μm, preferably 2 to 7 μm. A content of diborontrioxide is 0.005 to 1.0% by weight, preferably 0.05 to 0.7% by weight,more preferably 0.1 to 0.6% by weight. A mixing amount of diborontrioxide is 10 to 1000 ppm, preferably 10 to 300 ppm, more preferably100 to 200 ppm, based on the amount of the polyacetal resin.

In the present invention, in order to impart necessary properties to theabove polyacetal resin to be used for molding according to itsapplication and object, known additives such as a polyamide, astabilizer described in Japanese Patent publication Unexamined No.247247/1990, a hindered phenol antioxidant, an ultraviolet absorber, ahindered amine weathering agent, an antistatic agent, a parting agent, alubricant, an inorganic filler and the like can be added in the rangewhich the purpose of the present invention is not impaired.

Time of crystallization of a mixture comprising the polyacetal resin,boron nitride and, if necessary, the additive, to be used for molding ispreferably 100 seconds or less. Time of crystallization in the presentinvention is defined as follows. 5 mg of a sample of the polyacetalresin mixture is heated to 200° C. at a rate of 320° C./min. with a DSC,it is kept at 200° C. for 2 minutes and then, the sample is cooled to150° C. at a rate of 80° C./min. Time of crystallization is the period(seconds) between the time when a temperature of a sample holder becomes150° C. and the time when the top of a heat generation peak appearsbecause of crystallization of the polyacetal resin.

When a polyacetal resin mixture having time of crystallization of morethan 100 seconds is used, a molded article having excellent dimensionalstability is not easily obtained. Time of crystallization is morepreferably 10 to 70 seconds, most preferably 20 to 50 seconds.

The above polyacetal resin mixture is molded by injection molding into amolded article having a maximum wall thickness of 0.2 to 1 mm,preferably 0.25 to 0.6 mm. With respect to an injection molding method,conventional methods can be used. However, preferable injection moldingmethods are (1) a method having a mold temperature of 100° to 150° C.,preferably 110° to 140° C. and (2) a method having a filling time of 1second or less, preferably 0.5 second or less, more preferably 0.1second or less.

The most preferable injection molding method is a method having a moldtemperature of 100° to 150° C. and a filling time of 0.1 second or less.

It is observed with an electron microscope that the thin-walled moldedarticle obtained by this invention has a maximum spherulite size of thepolyacetal resin of 40 μm or less and a thickness of an amorphous layer,which is a skin layer of the molded article, of 200 μm or less.

The maximum spherulite size is defined as follows. A slice having athickness of 10 μm is cut out of the molded article with a microtome. Anabout 340 magnification photograph of the crystalline part of thepolyacetal resin molded article is taken with a transmission typepolarized microscope using a crossed nicol. In case of diameters ofspherulites being clearly identified (FIG. 3), the maximum spherulitesize is the diameter of the largest spherulite among spherulites in thephotograph. In case of diameters of spherulites not being clearlyidentified (FIG. 2), the maximum spherulite size is a value of twice thelength of the maximal polarized monochrome parts of the spherulite amongspherulites of which a polarized monochrome part (a part having the samecolor) does not connect with a polarized monochrone part of otherspherulites and which are confirmed as particulate (the maximum lengthof a polarized monochrome part is regarded as the half of the spherulitediameter).

The amorphous layer is a layer, in which the spherulite is not found inthe above polarized photomicrograph.

Among thin-walled molded articles obtained by the present invention, amaximum spherulite size of a molded article having good dimensionalstability is 6 to 30 μm, more preferably 10 to 20 μm.

An amorphous layer thickness of the molded article having gooddimensional stability is 50 μm or less, more preferably 5 μm or less.

According to the present invention, the molded article of the polyacetalresin having little warpage at a high temperature and humidity can beobtained.

The present invention is suitable for molding of a thin-walled gear, athin walled cam, a thin-walled cam gear, a shutter and the like, whichare used for the application requiring thin wall thickness and accuracy.Among the above molding, the present invention is suitable for moldingof a shutter of a cartridge for a disk or tape.

The shutter of the cartridge for the disk or tape is a shutter attachedto the outer face of a case having a built-in magnetic disk or tape inorder to open and close a window for introducing a head.

Particularly, the present invention is most suitable for molding of ashutter of a floppy disk having a maximum wall thickness of 0.35 mm orless.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photomicrograph showing one example of resin crystallinestructure of a molded article of the present invention.

FIG. 2 is a photomicrograph showing another example of resin crystallinestructure of a molded article of the present invention.

FIG. 3 is a photomicrograph showing resin crystalline structure of amolded article obtained in Comparative Example.

FIG. 4 is a perspective view of form of a molded article, whosedemensional change and the like are measured in the present invention.

DESCRIPTION OF SYMBOLS

A: line caused by pasting photographs together

1: grain surface

2: mirror surface

a: angle

b: angle

BEST MODE FOR CARRYING OUT THE INVENTION Examples 1 to 19 andComparative Examples 1 to 21

Powders of polyacetal homopolymers acetylated at both ends were producedaccording to a method described in U.S. Pat. No. 2,998,409. MFRs of thepolyacetal homopolymers were 30 to 110 g/10 min. under ASTM D1238-57 Econdition. The powders of the polyacetal homopolymers were dried at 80°C. for 3 hours. 0.2% by weight of IRGANOX 245 manufactured by Ciba-GeigyLtd. as a antioxidant and at least one selected from boron nitride,talc, calcium carbonate and nylon 6,6 were incorporated into theresultant powder as shown in Tables 1 to 5. The resultant mixture wasmelt kneaded with a twin-screw vented extruder having a temperature of190° C., a screw revolution speed of 50 rpm, a discharge amount of 3kg/hour. After pellets cut with a cutter were dried at 80° C. for 3hours again, a molded article shown in FIG. 4 was produced with a 3ounce injection molding machine having a mold temperature of 110° C. anda filling time of 0.2 second. The molded article has a shape formed bybinding two boards having a size of 5 cm×5 cm at each one side, andhaving an angle of 90° between two boards and two outer surfacessubjected to processing of GR003, which is a grain pattern of TanazawaHakko-sha. The wall thickness of the molded article can be changed inthe range of 0.1 to 5 mm.

After molding, the molded article was left at 23° C. and a humidity of50% for 2 days, further left under an atmosphere of a high temperatureof 60° C. and a high humidity of 80% for 6 hours and finally left underan atmosphere of 23° C. and a humidity of 50% for 1 hour. Before puttingthe molded article under an atmosphere of 60° C. and a humidity of 80%,angles a and b of the molded article were shown in FIG. 4. The angles aand b, which became the largest angles within 1 hour after taking themolded article out of an atmosphere of 60° C. and a humidity of 80%, andputting it under an atmosphere of 23° C. and a humidity of 50%, wereangles a' and b' respectively. The larger of (a'--a) and (b'--b) wasmeasured as a maximum warpage angle. Before putting the molded articleunder an atmosphere of 60° C. and a humidity of 80%, the maximumspherulite size, the maximum thickness of the amorphous layer, whichwere determined among those measured on 5 spots of the molded article,and time of crystallization were measured according to the abovemethods.

These results are shown in Tables 1 to 5.

Example 20

A copolymer of an acetal comprising 2.8% of ethylene oxide was preparedaccording to a method described in U.S. Pat. No. 3,027,352. The MFR ofthe polymer was 40 g/10 min. under ASTM D1238-57 E condition. The acetalcopolymer was dried at 80° C. for 3 hours. 0.2% by weight of IRGANOX245manufactured by Ciba-Geigy Ltd. as antioxidant and 100 ppm of boronnitride having a content of 0.1% by weight of diboron trioxide wereincorporated into the dried acetal copolymer. A molded article having amaximum wall thickness of 1 mm was obtained according to the same methodas in Example 1.

Warpage of the molded article obtained in this way was measuredaccording to the same method as in Example 1. The warpage was 0.4°.

The maximum spherulite size of the polyacetal resin composing the moldedarticle was 25 μm. The thickness of the amorphous layer was 6 μm. Timeof crystallization was 70 seconds.

Example 21

A shutter having a maximum wall thickness of 0.35 mm was molded with amold for a shutter of a 3.5 inch floppy disk by using the same polymermixture as used in Example 3.

The shutter obtained with a 3 ounce molding machine having a moldtemperature of 100° C. and a filling time of 0.1 second was left underan atmosphere of a high temperature of 60° C. and a high humidity of 80%for 6 hours and finally left under an atmosphere of 23° C. and ahumidity of 50% for 1 hour. Warpage of 4 corners of the shutter wasmeasured according to the above method. An angle of the largest warpagewas less than 0.1°.

                                      TABLE 1                                     __________________________________________________________________________                     Polyacetal Molded article                                                     resin      Maximum wall                                                                          Time of                                                                              Maximum                                                                             Amorphous                         Type of additives except                                                                       MFR (g/10                                                                           thickness                                                                             crystallization                                                                      spherulite                                                                          layer Warpage                Example                                                                            for antioxidant (amount)                                                                  Type min.) (mm)    (sec.) size (μm)                                                                        (μm)                                                                             (degree)                                                                           Fig.              __________________________________________________________________________    1    Boron nitride (10 ppm)                                                                    Homo-                                                                              40    1       100    40    200   0.8  --                     (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              2    Boron nitride (50 ppm)                                                                    Homo-                                                                              40    1       70     30    50    0.4  --                     (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              3    Boron nitride (100 ppm)                                                                   Homo-                                                                              40    1       50     18     3    0.2  1                      (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              4    Boron nitride (200 ppm)                                                                   Homo-                                                                              40    1       20     10     3    0.3  --                     (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              5    Boron nitride (300 ppm)                                                                   Homo-                                                                              40    1       12      6     3    0.6  --                     (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              6    Boron nitride (1000                                                                       Homo-                                                                              40    1       12      6     3    0.8  --                     ppm) (B.sub.2 O.sub.3 : 0.1%,                                                             polymer                                                           average particle                                                              diameter: 2 μm)                                                       7    Boron nitride (10 ppm)                                                                    Homo-                                                                              10    1       80     28    150   0.9  --                     (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              8    Boron nitride (10 ppm)                                                                    Homo-                                                                              30    1       73     25    120   0.8  --                     (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              9    Boron nitride (10 ppm)                                                                    Homo-                                                                              40    1       70     16    30    0.3  --                     (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              10   Boron nitride (10 ppm)                                                                    Homo-                                                                              70    1       68     16     5    0.1  --                     (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              11   Boron nitride (10 ppm)                                                                    Homo-                                                                              100   1       67     10     2    0.1  2                      (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              12   Boron nitride (10 ppm)                                                                    Homo-                                                                              110   1       65      6     6    0.7  --                     (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                              13   Boron nitride (100 ppm)                                                                   Homo-                                                                              40    1       80     25    10    0.9  --                     (B.sub.2 O.sub.3 : 0.005%, average                                                        polymer                                                           particle diameter: 2 μm)                                              14   Boron nitride (100 ppm)                                                                   Homo-                                                                              40    1       70     25     7    0.9  --                     (B.sub.2 O.sub.3 : 1.0%, average                                                          polymer                                                           particle diameter: 2 μm)                                              __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                      Polyacetal Molded article                                                     resin      Maximum wall                                                                           Time of                                                                              Maximum                                                                             Amorphous                        Type of additives except                                                                       MFR (g/10                                                                           thickness                                                                              crystallization                                                                      spherulite                                                                          layer Warpage              Example                                                                             for antioxidant (amount)                                                                  Type min.) (mm)     (sec.) size (μm)                                                                        (μm)                                                                             (degree)             __________________________________________________________________________    15    Boron nitride (100 ppm)                                                                   Homo-                                                                              40    0.8      50     18    3     0.2                        (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                             16    Boron nitride (100 ppm)                                                                   Homo-                                                                              40    0.6      50     18    3     0.1                        (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                             17    Boron nitride (100 ppm)                                                                   Homo-                                                                              40    0.35     50     18    3     0.06                       (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                             18    Boron nitride (100 ppm)                                                                   Homo-                                                                              40    0.25     50     18    3     0.1                        (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                             19    Boron nitride (100 ppm)                                                                   Homo-                                                                              40    0.2      50     18    3     0.2                        (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                             __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                     Polyacetal Molded article                                    Com-             resin      Maximum wall                                                                          Time of                                                                              Maximum                                                                             Amorphous                    parative                                                                           Type of additives                                                                              MFR (g/10                                                                           thickness                                                                             crystallization                                                                      spherulite                                                                          layer Warpage                Example                                                                            for antioxidant (amount)                                                                  Type min.) (mm)    (sec.) size (μm)                                                                        (μm)                                                                             (degree)                                                                           Fig.              __________________________________________________________________________    1    --          Homo-                                                                              40    1       230    100   300   22   --                                 polymer                                                      2    Boron nitride (2 ppm)                                                                     Homo-                                                                              40    1       200    90    210   18   --                     (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                              3    Boron nitride (5 ppm)                                                                     Homo-                                                                              40    1       150    70    205   17   --                     (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                              4    Boron nitride (5 ppm)                                                                     Homo-                                                                              40    0.25    150    50    100   30   --                     (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                              5    Boron nitride (5 ppm)                                                                     Homo-                                                                              40    0.6     150    70    205   28   --                     (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                              6    Boron nitride (5 ppm)                                                                     Homo-                                                                              40    2       150    70    205   10   --                     (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                              7    Boron nitride (5 ppm)                                                                     Homo-                                                                              40    3       150    70    205    3   --                     (B.sub.2 O3: 0.1%, average                                                                polymer                                                           particle diameter: 2 μm)                                              8    Boron nitride (1100                                                                       Homo-                                                                              40    1        8      6     3     2   --                     ppm) (B.sub.2 O3: 0.1%,                                                                   polymer                                                           average particle                                                              diameter: 2 μm)                                                       9    calcium carbonate (1000                                                                   Homo-                                                                              40    1       100    70     10    5   3                      ppm) (avrage particle                                                                     polymer                                                           diameter: 1 μm)                                                       10   talc (1500 ppm)                                                                           Homo-                                                                              40    1        98    45    210    2   --                     (average) particle                                                                        polymer                                                      11   diameter: 5 μm) nylon                                                                  Homo-                                                                              40    1        95    45    210    3   --                     6,6 (5000 ppm) (average                                                                   polymer                                                           particle diameter: 4 μm)                                              __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                       Polyacetal Molded article                                                     resin      Maximum wall                                                                          Time of                                                                              Maximum                                                                             Amorphous                  Comparative                                                                          Type of additives except                                                                       MFR (g/10                                                                           thickness                                                                             crystallization                                                                      spherulite                                                                          layer Warpage              Example                                                                              for antioxidant (amount)                                                                  Type min.) (mm)    (sec.) size (μm)                                                                        (μm)                                                                             (degree)             __________________________________________________________________________    12     Boron nitride (10 ppm)                                                                    Homo-                                                                              30    2       73     25    120   3                           (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                            13     Boron nitride (10 ppm)                                                                    Homo-                                                                              30    3       73     25    120   4                           (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                            14     Boron nitride (10 ppm)                                                                    Homo-                                                                              30    0.15    73     10    70    2                           (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter: 2 μm)                                            15     Boron nitride (100 ppm):                                                                  Homo-                                                                              40    2       50     18    3     3                           (B.sub.2 O.sub.3 : 0.1%, avrage                                                           polymer                                                           particle diameter: 2 μm)                                            16     Boron nitride (100 ppm)                                                                   Homo-                                                                              40    3       50     18    3     4                           (B.sub.2 O.sub.3 : 0.1%, avrage                                                           polymer                                                           particle diameter: 2 μm)                                            17     Boron nitride (100 ppm)                                                                   Homo-                                                                              40    0.15    50     18    3     2                           (B.sub.2 O.sub.3 : 0.1%, avrage                                                           polymer                                                           particle diameter: 2 μm)                                            __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________                       Polyacetal Molded article                                                     resin      Maximum wall                                                                          Time of                                                                              Maximum                                                                             Amorphous                  Comparative                                                                          Type of additives except                                                                       MFR (g/10                                                                           thickness                                                                             crystallization                                                                      spherulite                                                                          layer Warpage              Example                                                                              for antioxidant (amount)                                                                  Type min.) (mm)    (sec.) size (μm)                                                                        (μm)                                                                             (degree)             __________________________________________________________________________    18     Boron nitride (100 ppm)                                                                   Homo-                                                                              40    1       110    20    210   3                           (B.sub.2 O.sub.3 : 0.002%, average                                                        polymer                                                           particle diameter: 2 μm)                                            19     Boron nitride (100 ppm)                                                                   Homo-                                                                              40    1       120    30    210   4                           (B.sub.2 O.sub.3 : 1.2%, average                                                          polymer                                                           particle diameter: 2 μm)                                            20     Boron nitride (100 ppm)                                                                   Homo-                                                                              40    1       200    60    200   9                           (B.sub.2 O.sub.3 : 0.1%, average                                                          polymer                                                           particle diameter:                                                            11 μm)                                                              21     Boron nitride (100 ppm)                                                                   Homo-                                                                              40    1       200    50    210   5                           (B.sub.2 O.sub.3 : 0.1%, avrage                                                           polymer                                                           particle diameter:                                                            0.6 μm)                                                             __________________________________________________________________________

We claim:
 1. A method for producing a thin-walled molded article of apolyacetal resin comprising adding 10 to 1000 ppm of boron nitridehaving an average particle diameter of 1 to 10 μm and a content ofdiboron trioxide of 0.005 to 1.0% by weight to a polyacetal resin andmolding an article having a maximum wall thickness of 0.2 to 1 mm byinjection molding.
 2. The method of claim 1 wherein the thin-walledmolded article of the polyacetal resin is a shutter of a cartridge for adisk or tape.